11422 Discussion of Control and Protection

The objective of the protection and control system is to enable the distributed resource generators and/or storage devices to deliver the intended services to the users and the distribution system reliably, safely, and cost effectively. Protection, in the context of distributed resources connected to the utility network (i.e., operating in parallel), includes protection of network physical assets (lines, breakers, disconnects, transformers, etc.) and the utility line personnel, protection of the distributed resource assets and personnel, and protection of the loads served by the distributed resource in combination with the utility supply as it pertains to the flow of electrical power.

The distributed resource control function is focused on starting, stopping, paralleling, and disconnecting the generators and/or storage systems in an orderly and reliable manner. In addition, the controller monitors the health of the distributed resource subsystems such as generator over temperature, shutting down the units in an orderly manner should any fault or combination of fault conditions not related to the network connection, and delivery of power to or from the network occur.

Network protection and control systems have, until now, been designed for the central generation type of utility system, as described earlier. The introduction of distributed resources into these systems demands changes in the protection and control schemes of both the network and the distributed resource systems. In addition, the scope of the changes increases as the penetration of distributed resources, in terms of total number of units and total capacity, increases.

The need for additional protective devices and control logic is true even at the most fundamental level of DG. For example, a very small power-generating unit, such as a rooftop photovoltaic (PV) panel and inverter (perhaps a

* It is worth noting that even this very basic form of grid independent system can be very useful to the operation of the distribution system because, in principal, this specific load could be "shed" by remote control whenever necessary without any significant reduction in the power supplied to the load (of course, this depends on the duration of the load shedding).

few hundred W up to perhaps 3000 W peak capacity), is connected on the load side of a home's main distribution panel. Power flows both to and from the feeder on a net-metering basis. Such an installation requires anti-islanding protection logic and a transfer switch arrangement and associated control logic to ensure that there will be positive, immediate, and reliable disconnection from the feeder in the event of a feeder outage or an inverter/PV array fault.

While protection logic is part of the overall control logic, it is focused on solving two important problems:

1. Reducing potentially damaging transients when connecting and synchronizing the distributed generator units to the network and when disconnecting those units from the network

2. Protecting the utility feeder, the loads, and the utility personnel by ensuring that there is no possibility of one or more distributed generators continuing to supply a utility feeder and its loads following the disconnection of that feeder from the utility network (or, in the case of a general utility network outage, the occurrence of this unwanted continuing connection and supply of power to the feeder is termed islanding or "run on")

Transients are mitigated by the proper selection, installation, maintenance, and control of the transfer switch subsystem and the auto synchronizer. Both are highly standardized and mature products that can be readily selected for the specific distributed generator system.

As stated, the objective of the protection and control system is to enable the distributed resource generators and/or storage devices to deliver the intended services to the users and the distribution network reliably, safely, and cost effectively. The requirements (and, therefore, the complexity and cost) of protection and control systems for distributed resource systems, beyond the requirements of various standards, codes, and required certifications, depend primarily on:

• The size of the DG system with respect to the minimum total customer load on the feeder

• The number, size, and location of other DG units on the feeder

• The purpose of the DG — grid-connected or primarily grid-independent operating mode

• The type of DG — diesel generator, gas turbine generator, fuel cell, etc.

• The specific configuration of the feeder system (including laterals to the loads), including the size, location, operating mode, type of relays, breakers, and fuses, the feeder voltage, and the location, size, and configuration of all transformers

• Network operator requirements specific to that network (possibly as a result of experience with unique and unusual loads) and any additional safety requirements of local jurisdictions

Safety, in the broadest sense, pertains to (1) the safety of physical assets, e.g., the network, the distributed resource systems, and the loads served, and (2) the safety of humans, e.g., utility line personnel, the distributed resource operators and maintainers, and the operators of the end user's electrically powered equipment and infrastructure. The safety of personnel is mostly related to the potential for accidental or unintentional operation of a feeder, isolated from the network and powered by the distributed resources. Anti-islanding control logic and associated hardware devices designed specifically for the feeder characteristics are absolutely essential for distributed resource systems. The safety of utility line personnel is greatly jeopardized whenever anti-islanding cannot be ensured when the feeder is disconnected from the network.

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